Interrelationships between plant functional types and soil moisture heterogeneity for semiarid landscapes within the grassland/forest continuum: a unified conceptual model
Dd. Breshears et Fj. Barnes, Interrelationships between plant functional types and soil moisture heterogeneity for semiarid landscapes within the grassland/forest continuum: a unified conceptual model, LANDSC ECOL, 14(5), 1999, pp. 465-478
In semiarid landscapes, the ratio of herbaceous to woody plant biomass is a
major determinant of ecosystem properties. This ratio depends to a large e
xtent on the amount and spatial distribution of soil moisture that is avail
able to plants, and these variables, in turn, are determined primarily by c
limate and land use. Current conceptual models for determining the ratio of
herbaceous to woody plant biomass in semiarid plant communities are based
either on differences in soil moisture with depth (vertical heterogeneity)
from one site to another (Walter's two-layer model) or on differences in so
il moisture between canopy and intercanopy patches at the same site (horizo
ntal heterogeneity) that result from disturbances associated with land use
(Schlesinger et al.'s model of desertification). We developed a model that
unifies these two perspectives by relaxing two assumptions of Walter's two-
layer model. First, our model recognizes that soil moisture varies horizont
ally between canopy and intercanopy patches, not only due to land-use distu
rbance, a general assumption of the Schlesinger et al. model, but also due
to the physical nature of the canopy itself. Second, while retaining the ge
neral assumption of Walter that woody plants obtain moisture from deeper so
il layers than do herbaceous plants, our model recognizes the existence of
two types of woody plants: those that extract a substantial proportion of t
heir moisture from deeper layers and those that extract mainly from shallow
er layers. By modifying the two-layer hypothesis to include four soil compa
rtments and distinguishing between shallow- and deeper-rooted woody species
, our model integrates three key concepts in semiarid ecology: (1) the prop
ortion of woody cover increases as moisture in the deeper soil layers incre
ases (Walter's two-layer hypothesis for coexistence of herbaceous and woody
plants); (2) land use practices that cause a reduction in herbaceous veget
ation and compaction of intercanopy soils lead to a long-term increase in t
he proportion of woody plants (Schlesinger et al.'s concept, or more genera
lly, that at a given site multiple variations in the proportions of herbace
ous and woody plant biomass are possible); and (3) changes in the ratios of
herbaceous to woody plant biomass exhibit complex behavior (changes can ha
ppen quickly and are not directly reversible without intensive management).
This integration of concepts results because rather than assuming a simple
, one-way dependence of plant functional types on soil moisture heterogenei
ty, our model assumes an interdependence between the two: soil moisture het
erogeneity constrains the composition of the plant community, which in turn
modifies soil moisture heterogeneity. The four-compartment model that we p
ropose enables, for the first time, an integrated picture of both dimension
s of soil moisture heterogeneity - horizontal and vertical - and of the int
erdependence between soil moisture heterogeneity and the proportions of the
plant functional types that make up a given plant community. This unified
conceptual model can be applied to provide insight into the individual and
the combined effects of climate and land use on semiarid plant communities
within the grassland/forest continuum, which vary in the proportions of can
opy and intercanopy patches.